1. Field of the Invention
This invention relates to a photomask, a method of producing the same, a method of exposing using the same and a method of manufacturing semiconductor devices using the same. The invention can be utilized for photomasks used for forming various pattern images, a method of producing such photomasks, a method of exposing using such photomasks and a method of manufacturing semiconductor devices using such photomasks. For example, it can be utilized for photomasks used for the techniques of forming various patterns in semiconductor device manufacture processes, a method of producing such photomasks and a method of exposing using such photomasks. Further, it can be applied to an exposing apparatus and also to a method of semiconductor device manufacture, for instance a method of manufacturing such semiconductor devices as memory devices, logic operational devices, CCD devices, LCD devices, etc.
2. Description of the Related Art
The related art will now be described by taking the field of semiconductor devices as an example. When manufacturing a semiconductor device, various patterns are formed. In such semiconductor device manufacture, a pattern transfer process or commonly termed lithographic process is used mainly for transferring a photomask pattern on a resist material on a semiconductor wafer.
With a recent trend for finer semiconductor device structures, it is becoming more and more difficult to obtain a fine pattern with a desired resolution. As an example, the prior art photomask has a problem that the resist pattern size obtained by transfer is not in accord with a fine mask pattern size and is smaller than a desired value. Accordingly, in the prior art the mask pattern size is set to be greater than the resist pattern size that is obtainable by the transfer. Further, to solve the above problem and for the purpose of resolution improvement, investigations have been conducted about making shorter the wavelength of light of exposing in exposure apparatus used in the lithographic process, phase shifting masks for shifting the light phase, a shape change illumination process, in which the shape of a light source is changed, a pupil filter process, in which a filter is provided in an emission pupil in condenser lens system, and a FLEX process, in which exposing is done a plurality of times at different focus positions.
A general example of the prior art method will now be given. FIG. 1 is a graph illustrating the concept underlying the prior art method example. In this method, the transfer resist pattern size is determined by experiments or simulation with a plurality of different defocus values, thus obtaining a mask pattern size versus defocus curve 1. From this curve 1, the range of mask pattern size in a design tolerance range 2 is obtained. From this mask pattern size range depth of focus 3 is obtained, the numerical value of which indicates the performance of the lithography.
Further, there is a ED tree method which correlatively deals with the depth of focus and the exposure dose latitude. An example of this method is shown in FIG. 2. Curves 41 to 46 as shown, represent the relation between the exposure dose and the defocus for respective percentage changes in the transfer resist pattern size from the design mask pattern size. Assuming a design tolerance condition of the transfer resist pattern size that the percentage change therein from the design value is within .+-.10% (see curves 43 and 44 in FIG. 2) and that the exposure dose latitude that is necessary is 20% in range as shown by 5 in FIG. 2, the depth of focus is as shown by 3 in the Figure.
In simulation evaluation in various prior art techniques, the parameters that are evaluated are at most two in number. In cases where three or more parameters are evaluated, one of these parameters is fixed to a particular value, and only the relationship among the remaining two or more parameters is obtained. It is impossible to optimize three or more parameters under mutual correlation thereof.
In the prior art method of evaluation as exemplified above, no considerations are given to the fluctuations of the exposure dose and also the fluctuations of the mask pattern size of the mask. Therefore, the depth of focus that is obtainable is greatly aloof from the actual process condition, and it is greater than the actual depth of focus. Besides, in this method it is impossible to obtain quantitative evaluation of other parameters such as the exposure dose latitude and the mask pattern size latitude.
In the ED tree method (FIG. 2), again no considerations are given to the flucuations of the mask pattern size on the mask. In this case, there is a fatal drawback that it is impossible to obtain the mask pattern size latitude in addition to the serious drawback that the calculated depth of focus is extremely aloof from the actual process condition.
In the prior art method, the evaluation by simulation required an enormous amount of experiments for making up for the great aloofness from the actual process condition. The experiment requires time and cost, and in this case it is difficult to obtain efficient and systematic evaluation. Particularly, in the distal device development without established apparatus or material, there is a serious drawback that it is very difficult to find out the relationship among various techniques.
Further, as patterns are finer, a serious problem is posed by adverse effects of mask pattern size fluctuations on the transfer pattern, which has previously been any problem. In the prior art, it has been impossible to carry out evaluation by taking the mask pattern size fluctuations into consideration. Giving no consideration to the mask pattern size fluctuations means that the evaluation has heretofore been made under the assumption that the mask pattern size of the mask is in accord with the design mask pattern size and is fixed at all times. Actually, however, there are fluctuations in the mask production process, and it is impossible to perfectly eliminate the mask pattern size fluctuations. Hence, it has been indispensable for proper condition setting to let the mask pattern size fluctuations be reflected on the evaluation, but this has heretofore been impossible.